In the fabrication of buried heterostructure semiconductor lasers, a series of doped epitaxial layers are grown on a semiconductor substrate to define a heterostructure. Regions of the epitaxial layers are etched back to define an isolated mesa in the heterostructure which functions as an active region of the completed laser. The etched regions are covered with isolating material to planarize the structure, and contacts are formed to the active region and to the substrate. End facets are cleaved on the active region to complete the laser fabrication.
The isolating material used to planarize the structure must be compatible with the crystal structure of the heterostructure to avoid mechanical stresses leading to reliability problems and to avoid the formation of surface states which could act as recombination centers reducing the efficiency of the completed laser. The isolating material must also have a refractive index lower than that of the active region of the heterostructure so that the active region acts as an optical waveguide. Moreover, the isolating material must be a relatively poor conductor since current flow through the isolating material to the substrate would limit the flow of injection current through the active region of the heterostructure. The process used to cover the etched regions with isolating material must also be compatible with the crystal structure of the heterostructure to avoid degradation of the heterostructure during planarization.
Dutta et al have filled channels isolating a InP-InGaAsP buried heterostructure laser by masking regions adjacent the channels with a layer of SiO.sub.2, and selectively growing Fe-doped InP in the exposed channels using metalorganic vapour phase epitaxy (MOVPE) growth processes (Appl. Phys. Lett. 48 (23), pp. 1572-1573, June 9, 1986; Appl. Phys. Lett. 50 (11), pp. 644-646, Mar. 16, 1987). The Fedoped InP acts as a semi-insulator to inhibit flow of current through the filled channels, and has the required refractive index and crystal compatibility properties. However, Nakahara et al have shown that MOVPE-grown Fe-doped InP contains Fe-P precipitates which may degrade the long term reliability of such lasers (J. Crystal Growth 72, p.693, 1985). Moreover, unlike Liquid Phase Epitaxy (LPE) growth processes, MOVPE growth processes tend to produce a nonplanar epitaxial layer.
Unfortunately, LPE growth of Fe-doped InP requires temperatures exceeding 850 degrees Celsius to obtain adequate solubility of the Fe dopant in the InP melt, and temperatures exceeding 650 degrees Celsius damage the heterostructure to be isolated.
Rezek et al reported LPE growth of Co-doped semi-insulating InP at temperatures as low as 635 degrees Celsius (Appl. Phys. Lett. 43 (4), pp. 378-380, Aug. 15, 1983). However, later reports indicated that the LPE growth of Co-doped and Ni-doped semi-insulating InP is plagued by the formation of Co-P and Ni-P precipitates respectively (Proceedings of the 3rd NATO Workshop on Material Aspects of InP, Harwichport Mass., 1986).
Lambert et al reported growth of Ti-Hg co-doped semi-insulating InP ingots by a gradient freeze method (Semicond. Sci. Technol. 2 (1987), pp.78-82). The gradient freeze method is used to grow crystal ingots and is not applicable to the selective epitaxial growth of crystal layers on a substrate since the high melt temperatures used would decompose the substrate. Iseler et al reported growth of Ti-Zn, Ti-Cd and Ti-Be codoped semi-insulating InP boules by the Liquid Encapsulated Czochralski (LEC) method (Appl. Phys. Lett. 48(24), pp.1656-1657, June 16, 1986). The LEC method also is not applicable to the selective epitaxial growth of crystal layers on a substrate since the high melt temperatures used would decompose the substrate.